Hydrometallurgical processes for the recovery of metal values from low grade ore are increasingly important to the mining industry. Of particular interest is the hydrometallurgical recovery of copper and nickel. Typically an ore containing copper, nickel, and other valuable metals including chromium and cobalt as sulfide, oxide, or silicate salts is leached with aqueous sulfuric acid to form a dilute leach liquor containing the desired metal values. Wadsworth U.S. Pat. No. 3,868,439 describes one such process for leaching low grade copper ore with dilute sulfuric acid at a pH of 1.0-2.0. The metal values are then concentrated and purified into a form suitable for the recovery of the metals by conventional means.
Extraction of hydrometallurgical leach liquors with liquid extractants can be used to recover the metal values as described for example by Watson, Mod, and Teumac U.S. Pat. No. 3,475,163 or Skarbo U.S. Pat. No. 3,853,725 or U.S. Pat. No. 3,855,090. However, solvent losses make this technology less attractive for leach liquors containing a low level of metal values. Such losses can be eliminated by use of a solid ion exchange resin as an extractant, yet conventional ion exchange and chelate resins do not have the selectivity and activity essential for processing acid leach liquors.
Recently it has been discovered that chelate resins containing 2-picolylamine groups pendent to a cross-linked polymer matrix show particular promise as selective solid extractants for acidic leach liquors. They have a combination of desirable properties including capacity, kinetics, selectivity, and stability. Particularly important is the selectivity of these resins for removing valuable heavy metals including copper and nickel in the presence of ferric iron at a pH between about 1-5. The adsorbed metal values can be eluted with 1-10 N sulfuric acid with high recovery and a 5-10 fold decrease in iron content. However, for some applications such as direct electrolysis, an even lower iron content may be required thus necessitating further treatment.
Ferric iron is also detrimental to conventional precipitation or cementation operations. Reduction to ferrous iron with such materials as SO.sub.2 has been used although such reductions are often slow. Goren U.S. Pat. No. 3,109,732 and Spedden e.a. U.S. Pat. No. 3,669,651 use activated carbon as a catalyst for treating acid leach liquors with SO.sub.2. In Spedden & Malouf U.S. Pat. No. 3,819,797 SO.sub.2 or CO is injected below the surface of the ore body being leached to reduce residual Fe.sup.+.sup.3 prior to further processing.